After Retopology, should I apply the modifiers to have a good "Normal Map Bake"?
Assuming wireframe vertices on screenshot are from low poly retopology object, that object is not "sticking" to the high poly by itself. It's only through modifiers (mostly shrinkwrap) that it doesn't look like the vertices you can see around high poly. So you have to apply modifiers before baking (thought you should make a duplicate before that, to change modifiers in case something goes wrong).

After that the easiest you can do is use Smart Unwrap, it will separate your retopo into "random" parts unwraping it. Thought it would be quite a bit better if you marked seams (Ctrl+E->M) and Unwraped it yourself (U->Unwrap). You can look up some simple tutorials on that (this from BlenderGuru for example).